[fetch] Consider allow Service Worker to pass-through the original referrer (#89)

Filing a bug as recommended at https://github.com/slightlyoff/ServiceWorker/issues/404#issuecomment-89755099

Some SW users want to "pass-through" the original referrer, e.g., the following code wouldn't override the referrer of the original request: 
``onfetch = function(e) { e.respondWith(fetch(e.request)) }``

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/89